GCD Software

02. Building DEMs

Having good DEMs is critical for effective change detection. In this series of video tutorials, we lay out how to build DEMs from raw topographic data (e.g. rtkGPS or total station data). A similar procedure is available for airborne LiDaR data here.

Overview

In
this tutorial we are provided with some raw topographic survey data. Your
job is to build a DEM from that data (of appropriate resolution), show
the water depth overlaid on the DEM, and pull a longitudinal profile and
some cross sections off the DEM. You will use a triangular irregular
network (TIN) to interpolate between your raw topographic survey data
and produce a continuous surface that you will later convert to a raster
DEM.

NOTE: These instructions are for ArcGIS 10 and are primarily
provided in the form of video tutorials. As an additional reference, you
might find the 'Using ArcGIS 9.3.X to Construct and Manipulate DEMs'
tutorial listed in the main Lab 6 page
helpful (it uses different data, and is for the old version of ArcGIS,
but goes through a similar sequence of steps to arrive at the same end
point in Part I of the document; one significant difference is the
absence of TIN editing in ArcGIS 9.3.X).

Although not covered in the video tutorial, I would suggest exporting the PatsCabinSurveyPoints.csvtable you imported as X-Y points to a shapefile or feature class to use in the construction of the TIN (I named mine TopoSurveyPoints).

Building TINS

Your First TIN

In
this video tutorial, we go through the simplest form of constructing a
TIN from just raw topographic survey point data. We also highlight some
of the potential pitfalls to this approach.

A Better TIN

In
this video tutorial, we talk about how you can build a TIN using a
polygon hardclip boundary as well as how to manually edit your tin with
the TIN Editing Tools.

A Better TIN yet

In
this video tutorial, we will use the same data used in our first two
TINs as well as the some 3D breakline data provided to you:

Building DEM

For
this exercise, we have not been careful with the creation of this raster (see best practices for a follow up).

OPTIONAL: Building Water Depth DEM

Since
we have a DEM that includes shots above the water and below the water,
and because the survey data denoted where the water's edge shots are, we
can derive a bathymetric water depth DEM. The way in which we will do
this is to:

Build a water surface TIN from the water edge data

Convert that water surface TIN into a DEM of the same resolution and extent (i.e. concurrent) as our original DEM

Use Raster Calculator to derive water depths by subtracting the lower elevation DEM from the higher elevation water surface.

The
resulting water depth map can be overlaid on the DEM and provides a
more intuitive context of the physcial habitat within the reach as the
pools, bars, and beaver ponds jump out more clearly then from the
topography alone. This video walks you through the sequence:

OPTIONAL: Pulling Some Cross Sections and a Long Profile Off DEM

Using
the 3D Analyst interpolate line and profile graph features, you can
easily pull off a longitudinal profile and cross section(s) from the
DEM. This video shows you how:

References

The above exercise comes from a Lab in my Advanced GIS Class. If you want more information on how to work with DEMs and topographic data, you may find Labs 6, 7 and 8 helpful references/refreshers.